docs: def top(self, topn, by='counts'):
          """
          Get the top ``topn`` features in the :class:`.FeatureSet`\.
  
          Parameters
          ----------
          topn : int
              Number of features to return.
          by : str
              (default: 'counts') How features should be sorted. Must be 'counts'
              or 'documentcounts'.
  
          Returns
          -------
          list
  
          """
  
          if by not in ['counts', 'documentCounts']:
              raise NameError('kwarg `by` must be "counts" or "documentCounts"')
  
          cvalues = getattr(self, by)
          order = argsort(list(cvalues.values()))[::-1][:topn]
          keys = list(cvalues.keys())
          return [(self.index[keys[i]], cvalues[keys[i]]) for i in order]